Method of production of polymer/metal or metal sulphide composites, which uses metal mercaptides

ABSTRACT

The method for the production of polymer/metal or metal sulphide composites comprises the stages of: preparing a dispersion of a metal mercaptide in a polymer, the metal of the mercaptide being selected from the group comprising the transition metals and earth metals, and heating the said dispersion so as to cause thermal decomposition of the mercaptide and obtain the corresponding metal or metal sulphide in the form of inclusions in a polymer matrix.

The present invention relates to a method of production of polymer/metalor metal sulphide composites, in particular nanocomposites consisting ofmetallic particles or particles of metal sulphides dispersed inthermoplastic polymer matrices.

There is at present considerable interest in polymer-metal andpolymer-semiconductor nanocomposites, the latter often based on metalsulphides (for example PbS, SnS, etc.), on account of the particularoptical, magnetic, electronic and catalytic properties that characterizethese materials (see: Mayer A. B. R., Mater. Sci. Eng. C6 (1998)155-166;Caseri W., Macromol. Rapid Commun. 21 (2000) 705-722).

By making composite systems consisting of metallic or semiconductornanoparticles dispersed in polymer matrices, it is possible to combinethe characteristics of materials that are profoundly different from oneanother. Properties such as magnetism, catalytic activity, electricaland thermal conductivity etc. of metals or semiconductors are combinedin this way with the low specific gravity, chemical inertness, easyprocessability and film-forming ability, reduced cost etc. of polymers.Careful selection of the two components results in composites with newand irreplaceable characteristics, with great potential for application,and that can be processed with technologies that are already well knownand are of low cost.

The possibility of using preparative techniques that make it possible tocontrol the quantity and average size of the metallic or semiconductorinclusions is extremely important for the development of thesematerials, since these parameters determine many of the finalcharacteristics of the material.

Polymer-metal and polymer-semiconductor composites can be obtained bytwo different preparative schemes. The first, called ex situ, envisagesthe chemical synthesis of the powdered metal or semiconductor, itssurface passivation and finally introduction in the polymer matrix. Thesecond technique, called in situ, envisages the introduction of aprecursor (for example an organic salt or an organometallic complex)into the polymer and its subsequent decomposition by means of radiantenergy (e.g. thermolysis, photolysis, radiolysis, ultrasound, etc.). Thefirst technique, although very laborious, proves suitable mainly formodifying optical plastics intended for example for making particularoptical devices (e.g. colour filters, polarizers, waveguides, etc.),since the optical purity of the resulting composite is very high, as thesystem does not contain any type of by-product. The first of the twopreparative approaches is also advisable in sectors requiring materialsthat do not release toxic molecules by diffusion (for example in thefood and biomedical sectors, etc.). The second technique and especiallythat based on the thermolysis of precursors that are introduced into thepolymer is suitable mainly owing to its simplicity and speed, and can beused where high chemical purity of the final product is not required. Inparticular, the in-situ techniques make it possible to produce the metalor semiconductor directly during the process of moulding of thethermoplastic polymer. The precursor must, however, be chemicallycompatible with the thermoplastic polymer so that it can be mixed withit directly and furthermore it must be able to decompose in conditionssuch that the structural stability of the polymer matrix is notcompromised. Obviously, the process conditions of the material must alsotake into account the thermal decomposition of the precursor.

In the in-situ technique, formation of the metallic or metal sulphidephase envisages three main stages: a first stage of decomposition of theprecursor with formation of free atoms or molecules, a second stage ofnucleation of the latter with formation of crystal nuclei and a thirdstage of growth of the nuclei formed. In particular, the atoms ormolecules, once produced, will, owing to their small dimensions, be ableto migrate easily by diffusion within the polymer matrix and, when theirconcentration reaches the nucleation threshold, phase separation will beobserved. The nuclei produced, being of larger dimensions, will be muchless mobile and essentially will increase in size by surface depositionof further atoms and molecules present in the system. The number andfinal size of the particles will depend on the quantity of precursor andon the process conditions, whereas their shape will be approximatelyspherical.

Although there is a certain number of organic compounds that can undergothe process of thermal decomposition in conditions compatible with thechemical stability of the polymer, a class of substances permitting thegeneration of various types of metallic inclusions (transition metalsand earth metals, as well as their sulphides) has not yet been describedin the literature.

For their part, metallic powders of molybdenum, cobalt and iron havebeen widely used in the past for desulphurization of gasoline and thedecomposition of the resulting mercaptides for the production ofartificial sulphur (see: C. M. Friend, D. A. Chen, Polyhedron,16(18)(1997)3165). This process has been exploited industrially to asmall extent for the production of powders of transition metals or theirsulphides. Moreover, the use of paints based on mercaptides (resinates)of gold has been proposed in the past for the metallization of ceramicand vitreous substrates (U.S. Pat. No. 6,231,925 of 1 Aug. 1961 in thename of Howard et al.; U.S. Pat. No. 5,707,436 of 13 Jan. 1998 in thename of Fritsche et al.; U.S. Pat. No. 2,984,575 of 16 May 1961 in thename of Howard et al.; U.S. Pat. No. 2,490,399 of 6 Dec. 1949 in thename of Kermit et al.). Metallization with resinates has also beenproposed for other metals (U.S. Pat. No. 4,808,274 of 28 Feb. 1989 inthe name of Nguyen). Only recently, polymercaptides have been used forthe synthesis of minute clusters of gold and palladium (T. G. Shaaff, M.N. Shafigullin, J. T. Khoury, I. Vezmar, R. L. Whetten, W. G. Cullen, P.N. First, C. Gutierrez-Wing, J. Ascensio, M. J. Jose Yacamán, J. Phys.Chem. B 101(40) (1997) 7885-91). Occasionally, mercaptides of tin havebeen used as additives for polyvinylchloride (PVC) as internallubricants (U.S. Pat. No. 5,371,149 of 6 Dec. 1994 in the name ofKishida et al.).

The object of the present invention is to supply a method for theproduction of polymer/metal or metal sulphide composites, using asprecursors a class of chemical compounds with which it is possible toobtain metallic or sulphide inclusions with magnetic, catalytic etc.properties that vary significantly depending on their nature.

According to the invention, this aim is achieved by means of a methodfor the production of polymer/metal or metal sulphide composites,comprising the stages of:

-   -   preparing a dispersion of a metal mercaptide in a polymer, the        metal of the mercaptide being selected from the group comprising        the transition metals and the earth metals, and    -   heating the said dispersion so as to cause thermal decomposition        of the mercaptide and obtain the corresponding metal or metal        sulphide in the form of inclusions in a polymer matrix.

The mercaptides (also called thiolates) are organosulphur compoundswhose structure consists of a metal atom bound to one or more sulphuratoms each bearing an organic group, for example alkyl or aryl. Thesecompounds can be regarded as products of salification of thecorresponding mercaptans (or thiols) and can be represented by theformula Me(SR)_(n), where Me indicates a transition metal or earthmetal, R indicates an organic group and n is an integer that correspondsto the valency of the metal.

Preferably the metal is selected from the transition metal group(groups: IIIB, IVB, VB, VIIB, VIIB, IB and IIB of the periodic table ofthe elements) and of the earth metals (group IIIA of the periodic tableof the elements), while the organic group is selected from thelong-chain linear aliphatic hydrocarbons (normal alkyl groups,—C_(n)H_(2n+1), with n>10).

The weight ratio in the dispersion between metal mercaptide and polymercan be advantageously between 0.01 and 0.2, and preferably between 0.05and 0.1. The dispersion is heated advantageously at a temperaturebetween 100 and 500° C., and more preferably between 150 and 250° C.

An advantageous property of the mercaptides derived from long-chainalkane thiols and transition metals or earth metals is that they arehydrophobic compounds, soluble or at any rate readily dispersible innonpolar organic solvents (both aliphatic and aromatic hydrocarbons,chlorinated hydrocarbons, ethers, etc.). The hydrophobic nature of thesecompounds makes them absolutely compatible with many engineeringpolymers, with which they form homogeneous systems (solid solutions)even at high concentrations. The hydrophobic nature of these compoundsis due to the fact that the Me—S bond is heteropolar covalent and thatthe partial charge present on the two elements is very small on the onehand because of the reduced electronegativity of the sulphur atom and onthe other hand because of the moderate electropositivity of the metal.Moreover, the remaining part of the molecule is nonpolar owing to thepresence of extensive hydrocarbon residues bound to the sulphur atoms.

Furthermore, the method of the invention exploits advantageously themoderate thermolability of the mercaptides of transition metals and ofearth metals. In fact, these compounds decompose quantitatively underthe action of heat (thermolysis), releasing metal atoms at temperaturesbetween 100° C. and 300° C. in accordance with the following well-knownreaction scheme (in which, for simplicity, reference is only made to adivalent metal):Me(SR)₂→Me+R—S—S—R

In some cases (when for example Me=Sn or Pb) thermal decomposition ofthe mercaptide leads to the production of metal sulphides:Me(SR)₂→MeS+R—S—R

These sulphides can decompose further at higher temperatures, givingrise to metal and sulphur. The temperatures required for thermalhomolysis can be controlled by varying the nature of the organic groupR, and thus become compatible with the thermal stability of the polymersand superimposable with those usually employed in their processing inthe molten state.

The mercaptides employed in the method of the invention have the furtheradvantage that they can be synthesized chemically in an extremely simplemanner and at a quantitative yield. In particular, it is possible toprepare an alcoholic and if necessary alkaline solution (for examplesodium or potassium hydroxide in ethanol) of the correspondingmercaptan. An organic or inorganic salt of the transition metal or earthmetal (for example acetate, chloride, nitrate etc.) is then added tothis solution. The solubility of the salts in alcohol is generally veryhigh. Then, the metal mercaptide is separated from the reaction mediumin the form of precipitate, exploiting its nonpolar nature. Thesereactions can be represented by the following formulae, where Xindicates the anion of the salt:MeX_(n)→Me^(n+)+nX⁻(dissolution of the salt)Me^(n+)+nR—S⁻→Me(SR)_(n)(precipitation of the mercaptide)

Owing to the presence of unoccupied orbitals on the metal atom (d and forbitals) and of lone electron pairs on the sulphur atoms, themercaptide of transition metals generally constitutes polymericstructures of the type:mMe(SR)_(n)→poly-Me(SR)_(n)(polymerization of the mercaptide)

The polymeric nature of the mercaptides of transition metals is alsovery advantageous because it facilitates the dispersion of themercaptide in the molten polymer, which can take place directly duringthe operations of shaping of the latter.

The polymer used in the method of the invention can be one of thosecommonly employed in industry, provided it can with-stand thetemperatures necessary for inducing thermolysis of the mercaptide. Forexample, the said polymer can be selected from the group comprisinghigh-performance thermoplastic polymers (PEEK, PPO, Ultem, capton,etc.), optical plastics (PS, PC, PMMA, etc.) and common engineeringpolymers (PE, PP, PET, etc.).

Without wishing to be bound to a specific theory, it is possible tohypothesize that the particular mechanism involved in the reaction ofthermal decomposition of the metal mercaptide as well as the thermalthreshold necessary for it to start are closely connected with thenature of the organic groups present in the mercaptide and with theirsize.

In particular, heat treatment can cause homolytic splitting of themetal-sulphur bonds, of the sulphur-carbon bonds, or of a metal-sulphurbond and a sulphur-carbon bond. In the first case the thermolysis of themercaptide will lead to the generation of metal atoms and hence, at theend of the procedure, atomic clusters of metal will be contained in thepolymer matrix. In the second and third case, however, molecules ofmetal sulphide will be generated and therefore the composite producedwill contain molecular clusters of metal sulphide. In this connection,it should be pointed out that the second case is essentially atheoretical construction, since it envisages the simultaneous formationof two highly unstable organic radicals and moreover would require thestability for the metal of high states of oxidation (for example +4).

An appropriate choice of the organic group R and in particular of itscapacity for stabilizing the radical generated by the thermal homolysisof the bond with sulphur can make it possible to direct the thermolysistowards the formation of the metal or of the sulphide. In fact, if theorganic group is capable of stabilizing the corresponding radical,formation of the sulphide will be favoured, otherwise the metal will beobtained. Therefore, groups such as benzyl or allyl, which providestrong resonance stabilization of the corresponding radicals, are ableto favour the formation of the metal sulphide. The n-alkyl groups, onthe other hand, can stabilize the corresponding radicals only by meansof mild hyperconjugative effects and therefore thermolysis will be ableto generate both sulphide molecules and metal atoms. The size of thealkyl group will be determining in this case. In this way, metal atomswill be obtained with lower alkyl groups, for example containing up totwelve carbon atoms, on account of their high diffusivity in the moltenpolymer and hence their possibility of moving away once generated. Incontrast, the metal sulphide will be obtained with higher alkyl groups,i.e. having more than sixteen carbon atoms. Finally, branched alkylgroups (e.g. tert-butyl, isopropyl) should favour the formation of themetal relative to the sulphide on account of their absolute inability tostabilize the corresponding radical.

The temperature required for thermal decomposition of the mercaptidealso depends on the nature of the organic group R and in particular onits size. As a rule, the larger the alkyl group R, the higher is thetemperature necessary for thermolysis. This might be explained on thebasis of the greater quantity of kinetic energy that the molecularfragments must possess in order to be able to move away from one anotherafter scission, avoiding recombination.

Consequently, it is possible to select the organic group R to combinewith a given metal, so as to make the temperature of thermolysis of themercaptan, as well as the nature of the compounds that are derived fromit, more suitable in the light of the specific requirements.

Other advantages and characteristics of the present invention willbecome clear from the following examples of application, which are giventhough without constituting any limitation as to the nature of themercaptide. The description of these examples also refers to theappended drawings, in which:

FIG. 1 is a TGA thermogram of the metal mercaptide (palladiumdodecanethiolate) used in example 1,

FIG. 2 is a TEM micrograph of the composite material (nanoparticles ofelementary palladium in polystyrene) obtained at the end of example 1,

FIG. 3 is a TGA thermogram of the metal mercaptide (leaddodecanethiolate) used in example 2,

FIG. 4 is a TGA thermogram of the metal mercaptide (cobaltdodecanethiolate) used in example 3, and

FIG. 5 is a TEM micrograph of the composite material (nanoparticles ofelementary cobalt in polystyrene) obtained at the end of example 3.

EXAMPLE 1

Palladium nitrate was dissolved in ethanol and dodecanethiol was addedto the solution, while stirring. The yellow precipitate of palladiumdodecanethiolate obtained was separated by centrifugation, washedseveral times with ethanol or acetone and finally dissolved in hot (50°C.) chloroform and re-precipitated by adding ethanol. The product wasseparated by centrifugation and then left to dry in the air. A solutionof palladium dodecanethiolate in chloroform was then added to a solutionof polystyrene in chloroform and the system, after thoroughhomogenization, was poured onto a glass surface and left to dry. In thisway, transparent films of a deep yellow colour were produced, and thesewere then submitted to heat treatment at 340° C. by means of a sandbath. A thermogravimetric analysis (TGA) was carried out on thepalladium dodecanethiolate, as shown in FIG. 1. The thermogram revealstwo stages in degradation: the first is determined by the thermaldecomposition of the palladium mercaptide which leads to the formationof the corresponding sulphide, and the second is further decompositionof the sulphide, from which the metal is obtained. The microstructure ofthe inclusions of Pd in polystyrene was visualized by examination bytransmission electron microscopy, shown in FIG. 2.

EXAMPLE 2

Lead dodecanethiolate was produced following the procedure describedwith reference to example 1 and using hydrated lead perchlorate as thestarting salt. Slightly opaque films of a deep yellow colour wereobtained after dispersing the mercaptide in polystyrene. The films wereheat-treated at 200° C. to produce inclusions of lead sulphide and at350° C. to produce inclusions of metallic lead, by means of a sand bath.The TGA thermogram of the lead dodecanethiolate is shown in FIG. 3.

EXAMPLE 3

Cobalt dodecanethiolate was produced following the procedure describedwith reference to example 1 and using cobalt chloride as the startingsalt. The films of polystyrene-cobalt dodecanethiolate were perfectlytransparent, with a deep red colour. These films were then submitted toheat treatment at 200° C. using a sand bath. The TGA thermogram of thecobalt dodecanethiolate is shown in FIG. 4 and the microstructure of theinclusions of Co is shown in FIG. 5.

EXAMPLE 4

Copper dodecanethiolate was produced following the procedure describedwith reference to example 1 and using hydrated cuprous chloride as thestarting salt. The films of polystyrene-copper dodecanethiolate obtainedwere transparent and of a deep yellow colour. These films were thensubmitted to heat treatment at 200° C. using a sand bath.

Of course, without prejudice to the principle of the invention, thedetails of application and the embodiments can vary widely relative tothe foregoing purely illustrative description, without departing fromits scope as claimed.

1. Method for the production of polymer/metal or metal sulphidecomposites, comprising the stages of: preparing a dispersion of a metalmercaptide in a polymer, the metal of the mercaptide being selected fromthe group comprising the transition metals and the earth metals, andheating said dispersion so as to cause thermal decomposition of themercaptide and obtain the corresponding metal or metal sulphide in theform of inclusions in a polymer matrix.
 2. Method according to claim 1,in which said polymer is selected from the group comprisinghigh-performance thermoplastics, optical plastics and ordinaryengineering polymers.
 3. Method according to claim 1, in which saidmetal is selected from the metals belonging to the groups: IIIB, IVB,VB, VIIB, VIIB, IB, IIB or IIIA of the periodic table of the elements.4. Method according to claim 1, in which said mercaptide contains anorganic group selected from the long-chain, linear aliphatichydrocarbons.
 5. Method according to claim 1, in which the weight ratioin said dispersion between metal mercaptide and polymer is between 0.01and 0.2.
 6. Method according to claim 1, in which the heating of thedispersion is carried out at a temperature between 100° C. and 500° C.7. Method according to claim 1, in which said metal mercaptide isobtained as a precipitate that forms as a result of reaction of thecorresponding mercaptide in alcoholic solution with an organic orinorganic salt of the metal.
 8. Method according to claim 1, in whichsaid dispersion is prepared by mixing a solution of the metal mercaptidein a solvent with a solution of the polymer in the same solvent. 9.Method according to claim 1, in which said mercaptide contains alkylgroups C_(n)H_(2n+1) with n>10.
 10. Method according to claim 1, inwhich the weight ratio in said dispersion between metal mercaptide andpolymer is between 0.05 and 0.1.
 11. Method according to claim 1, inwhich the heating of the dispersion is carried out at a temperaturebetween 150° C. and 250° C.